1. Field of the invention
The present invention relates to a copper material useful as a superconductive material.
2. Related Background Art
Hitherto known as copper oxides are CuO and Cu.sub.2 O, as well as CuGeO.sub.3, CuNbO.sub.3, CuVO.sub.3, Bi.sub.2 CuO.sub.4, Cr.sub.2 CuO.sub.4, La.sub.2 CuO.sub.4, et. Of these, La.sub.2-x A.sub.x CuO.sub.4 (A=Ca, Sr or Ba) and LnBa.sub.2 Cu.sub.3 O.sub.y (Ln=Y or lanthanoids) are known as superconductive materials.
However, in the conventional superconductive materials comprising copper oxides, the yttrium and lanthanoids that are component elements of the materials have been expensive materials because of their small amounts of the resource. In instances in which these copper oxides are formed into sinters or thin films depending on what they are used for, high temperatures of about 1,000.degree. C. are required as reaction temperature, bringing about the problems that a high production cost results and there are considerable limitations on substrates to be used. Moreover, they also can be formed into single crystals with difficulty, and with restricted reaction conditions, so that no large single crystal has ever been obtained. Furthermore, deviation in compositional ratios may greatly affect the superconductivity transition temperature (hereinafter "Tc"), having brought about the problem that the materials exhibit no superconductivity Within the range of x.gtoreq.0.2 in, for example, Y.sub.1+x Ba.sub.2-x Cu.sub.3 O.sub.7. This is particularly a great problem since the compositional deviation becomes liable to occur When thin films are prepared.
Z. Phys. B-Condensed Matter 68, 421-423 (1987) discloses a novel Bi-based superconductive material, having its composition of Sr.sub.2 Bi.sub.2 Cu.sub.2 O.sub.7+.delta., Tc of 7 to 22 K in the midpoint. This Bi-based superconductive material does not employ any expensive raw materials such as Y and lanthanoids as its component elements, can be formed using reaction temperatures of not higher than 900.degree. C., can be inexpensive when compared with conventional La.sub.2-x A.sub.x CuO.sub.4 and LnBa.sub.2 Cu.sub.3 O.sub.y, and can afford to accept a broader scope of selection in respect of the materials for substrates when thin films are formed, and thus can be said to be superior materials in these respects. They, however, have the problems such that the Tc tends to be extremely lowered because of inclusion of impurities and it is difficult to obtain a superconductive material having a stable Tc.